A person who is Tamei (ritually impure) is not allowed to enter any spot of the Beit HaMikdash or Har HaBayit (the Temple Mount). But these places range in levels of holiness. A Korban (offering) is only obligatory if a person enters parts of the actual Beit HaMikdash while he is Tamei. But if he goes onto Har HaBayit, the Temple Mount, while he is Tamei, he is not obligated to bring a Korban. Here, the Mishnah is going to talk about which parts of the Beit HaMikdash obligate a Tamei person to bring a Korban when he enters them while Tamei.
The Mishnah says: A Korban is obligatory when somebody who is Tamei enters the Azarah, which is the main courtyard of the Beit HaMikdash, or an addition to the main courtyard. If area was later added to the courtyard, that added area takes on the sanctity of the Azarah itself. How do they add area to the Azarah, to the Temple Courtyard, or to Yerushalayim in general? The holiness of the Azarah is that certain Korbanot could be eaten there. A person who walked there would be obligated to bring a Korban. The sanctity of Yerushalayim is that certain types of Korbanot could be eaten there—lower-level Korbanot. Maaser Sheni (second tithe) could be eaten there as well, but not outside the city limits of Yerushalayim. So how do they add to the city or to the Azarah? What do they have to do? What is the formal process to get it done?
So we say: There has to be formal approval from a king and a Navi (a prophet), along with the Urim VeTumim. The Urim VeTumim was a piece of parchment that had the... written on it. It was placed into the Choshen, the breastplate of the Kohen Gadol (High Priest). When questions facing the nation had to be addressed, and the Jewish people needed to communicate with Hashem, they would ask the Urim VeTumim, and the stones on the Choshen would spell out Hashem’s answer. The Sanhedrin, the great Sanhedrin, also had to approve the expansion of either Yerushalayim or the Azarah. There also had to be two loaves of bread from the Todah offerings (Thanksgiving offerings), along with a song sung by the Leviim (Levites). The Todah, Thanksgiving offering, consisted of four loaves of bread. Thirty of them would be Matzah (unleavened bread), and ten of them would be Chametz (leavened bread). So when they were trying to build or enlarge in a city, they would bring two Todah offerings, for a total of 80 loaves. They took one Chametz loaf from each offering and brought them together. So now there were two Chametz loaves, and each loaf was carried by a Kohen (priest), who circled the entire area they wanted to add to Yerushalayim.
When talking about the expansion of the Beit HaMikdash, of the Azarah, they would not use a Korban Todah, but they would use a Korban Mincha (meal offering), which could be eaten in the temple courtyard, in the Azarah. The Korban Todah, by the way, could be eaten anywhere in Yerushalayim, so that is what they used. These loaves, like we said, would be marched around along the edge of the area they would be adding. Once they had circled the area, the addition became part of the original boundary of Yerushalayim or the courtyard, the Azarah, and all the holiness that that entails was joined with it.
How was this march? What did they do to make it happen? U'Veit Din, the great Sanhedrin, would march in the following manner. The two loaves from the two Todah offerings were carried all the way in front, and the Sanhedrin walked behind them. The King and the Navi and the Urim VeTumim and all that would not have to join. So the Sanhedrin would walk behind the two loaves. They are called Yisrael achareihem (Israel after them), and the rest of the Jewish people walked behind them, behind the Sanhedrin. They walked this way until they reached the end of the area they wanted to make holy.
After the march, these two loaves we had—the inner loaf was eaten, and the outer loaf was burned. There are different understandings of this according to the commentaries, but we will understand that the inner loaf was the one closer to the Sanhedrin, which was behind the loaves, and the outer one was the one up in front, all the way ahead. So the inner one would be eaten, and the outer one would be burned. Any addition to the Beit HaMikdash itself that was not done formally with all of these requirements—the approval of the King and the Navi, the asking of the Urim VeTumim, the great Sanhedrin, and that march with the Leviim—would not have the sanctity of the Beit HaMikdash. If somebody went there while he was Tamei, they would not be obligated to bring a Korban, because that spot they walked onto did not acquire the full holiness of the Beit HaMikdash.